Manufacture of a cellulose ester film according to a conventional solution cast film manufacturing process has been carried out employing an apparatus for manufacturing a cellulose ester film as shown in FIG. 1. FIG. 1 is a schematic view of a solution cast cellulose ester film manufacturing apparatus including a dissolution device. A manufacturing method of a cellulose ester film according to a conventional solution cast film manufacturing process will be explained below, employing FIG. 1. A cellulose ester is dissolved in an organic solvent in a pressure resistant vessel 1 (a dissolution step), filtered through a filter 2 to prepare a cellulose ester dope, transported through a supply pump 4 to a storing vessel 3, and cast on a metal support 5 through a supply pump 4 from a die 6 to form a web 7. The metal support 5 is a moving endless stainless steel belt support whose surface is subjected to mirror treatment or a drum support whose surface is plated with chromium. The web 7 is made semidry on the metal support, and then the semidry web 7 is peeled from the support through a peel roller 8 (located at a peel position). The peeled web 7 is introduced into a roller drier 9, transported through the guide rollers 10 during which drying of the web is completed, and finally wound around a spool 11 to obtain a cellulose ester film 12. In FIG. 1, the roller dryer 9 comprises many guide rollers 10 which are arranged so that the surface side and the rear side of the web 7 alternately contact the rollers. A numerical number 14 shows a condenser, and a numerical number 15 shows a material incorporation port. A tension changing device (not illustrated in FIG. 1) is located between the peel roller 8 and a spool 11 which changes the tension of the web upstream and downstream of the tension changing device.
Drying in the roller dryer is generally carried out employing a dry air, but there is also a method disclosed in U.S. Pat. No. 2,319,053 in which drying is carried out employing infrared rays. There is another drying method in which drying is carried out floating the web by air pressure and transporting the web in a non-contact state so that the web does not directly contact the guide roller (as disclosed in, for example, Japanese Patent O.P.I. Publication No. 55-135046).
Further, there is a tenter method in which a film such as a polyester film or a polypropylene film is stretched by a factor of 2 to 6, fixing the both side edges of the film. This tenter method can be also applied to a cellulose acetate film. A method of manufacturing a cellulose acetate film employing a tenter drying method is disclosed in Japanese Patent O.P.I. Publication Nos. 4-284211 and 62-115035.
FIG. 2 shows a schematic view of a solution cast cellulose ester film manufacturing apparatus comprising a tenter dryer (another device such as the dissolution vessel is not illustrated in FIG. 2). The web 7 peeled by a peel roller 8 is transported and dried in a tenter dryer 13 where both edges in the transverse direction of the web are held by a clip (not illustrated in FIG. 2) to maintain the width of the web constant-or the web is stretched a little. Roller dryers may be located upstream and downstream of the tenter dryer. In FIG. 2, roller dryer 9 is located downstream of the tenter dryer.
Recently, development has been made regarding decrease in thickness and weight of an information device with a liquid crystal display such as a note board type personal computer, a car navigation system, a cell phone, a game device, or a television. In parallel with this development, a cellulose ester film has been used as a protective film of a polarizing plate as an optical film or a film for electro-luminescence, and has been eagerly sought to be increasingly thinner.
However, when a thin cellulose ester film is installed in a liquid crystal display and the like, the film has the problem in that the film deforms at high temperature with time. In the manufacture of the thin film, when the web is peeled from a metal support or while the peeled web is transported, the web is likely to be influenced by outer force such as transport tension due to its softness, and to cause elongation or surface defects such as scratches, streaks and transverse streaks. The finished cellulose ester may contain air bubbles or foreign matter on account of insufficient defoaming or filtration in the dissolution-step of cellulose ester. The finished cellulose ester may contain undissolved matter which cannot be removed by a filter used. Such defects lower quality of the finished cellulose ester film, adversely affect a liquid crystal display, and result in cost increase or lowering of productivity.
In a thin cellulose ester film such as a cellulose ester film with a dry thickness of 20 to 60 μm, there arise problems such as deformation and occurrence of minute air bubbles which are undesired for a liquid crystal display and which have been hardly observed in a conventional thick cellulose ester film such as a cellulose ester film with a dry thickness exceeding 60 μm.